Coating composition for producing magnetically induced

ABSTRACT

The present invention is related to a coating composition for the production of a magnetically induced image, consisting of volatile components (S) and non-volatile components, the latter consisting of an ink vehicle (I) and magnetically orientable optically variable interference pigment (P), to a process for manufacturing the coating composition, and to the use of the composition for the production of a magnetically induced image coating on a substrate with the help of applied magnetic fields. Said magnetically induced image coating may be used as a security device on value- or identity documents, brand protection labels and the like.

RELATED APPLICATIONS

This application is a divisional of application Ser. No. 12/300,469filed Nov. 12, 2008, now U.S. Pat. No. 8,246,735, which is a nationalstage of PCT/EP2007/052993, filed Mar. 29, 2007, which claims priorityto European Application No. 06113891.3, filed May 12, 2006. Thedisclosures of application Ser. Nos. 12/300,469 and PCT/EP2007/052993are incorporated by reference herein in their entireties.

FIELD OF THE INVENTION

The present invention relates to coating compositions for producingmagnetically induced images. More specifically it relates to printinginks for producing magnetically induced images for use on security- orvalue-documents or on branded goods, to protect them against counterfeitor illegal reproduction.

BACKGROUND OF THE INVENTION

Optically variable devices of various types are used as efficientanti-copying means on security- and value-documents. Among these,particularly important optically variable copy-protection means are theoptically variable inks (OVI®; EP-A-0227423). Optically variable inks(OVI®) are used to print surfaces and/or indicia which exhibit a viewingangle dependent color (=color shift).

Said anti-copying inks are formulated on the basis of optically variablepigments (OVP); preferred types being the flake shaped thin-film opticalinterference pigments described in U.S. Pat. No. 4,705,300; U.S. Pat.No. 4,705,356; U.S. Pat. No. 4,721,271 and thereto related disclosures.

Other useful types of OVP for optically variable ink formulationscomprise the interference coated particles described in U.S. Pat. No.5,624,486 and U.S. Pat. No. 5,607,504, and the thin-film cholesteric(i.e. chiral-nematic) liquid crystal pigments described in U.S. Pat. No.5,807,497 and U.S. Pat. No. 5,824,733.

Optically variable inks, coatings and paints are known in the art, e.g.from EP-A-0227423, U.S. Pat. No. 5,279,657, or WO 95/29140. Saidoptically variable inks can be used in various printing processes, suchas copperplate (Intaglio) printing, gravure printing, flexographicprinting or screen printing.

As known to the skilled person, the wet film thickness resulting fromthe said printing processes can vary to a large extent, form about 2 μmup to about 50 μm depending upon the process and the conditions used.

To achieve a high color-shifting effect of the optically variable ink orcoating, the shape of the optically variable pigment (OVP) is preferablya platelet or a flake, as disclosed in the art.

The perceived optical characteristics and the color purity depend on thepigment's final orientation in the cured ink or coating layer on thesubstrate. Randomly oriented optically variable pigment flakes orplatelets exhibit a poor color shift and low color purity. Maximum colorshift and color purity requires the optically variable pigment flakes orplatelets in the ink or coating to adopt a same particular orientation,e.g. coplanar to the surface of the substrate.

These optical effects are even more enhanced if the surface of thesubstrate has been previously smoothened through the application of abase coating. The optically variable pigment flakes can in this casearrange more readily in a flat plane, thus increasing the coverage onthe substrate, the color purity and the color shift.

To obtain coatings having the optically variable pigment flakes arrangedin a same flat plane on a substrate, an ink or coating formulation whichallows for a reduction of the wet film thickness during the dryingprocess to less than 10 μm is normally used. The gradual reduction ofthe film thickness during the drying process forces the opticallyvariable pigment flakes to arrange in a single plane parallel to thesurface of the substrate, providing for a maximum coverage and colorshift on the substrate.

Magnetic optically variable pigments have been disclosed in WO02/073250; U.S. Pat. No. 4,838,648; EP-A-686675; WO 03/00801 and U.S.Pat. No. 6,838,166 as an improvement to the optically variable pigmentsfor inks on security-, value-documents and banknotes; these documentsare incorporated herein by reference.

Magnetic optically variable pigments in printing inks or coatings allowfor the production of magnetically induced images, designs and/orpatterns through the application of a corresponding magnetic field,causing a local orientation of the magnetic optically variable pigmentin the coating, followed by drying/curing the latter. The result is afixed magnetically induced image, design or pattern in an opticallyvariable ink.

Materials and technology for the orientation of magnetic particles incoating compositions and printing processes have been disclosed in U.S.Pat. No. 2,418,479; U.S. Pat. No. 2,570,856; U.S. Pat. No. 3,791,864; DE2006848-A; U.S. Pat. No. 3,676,273; U.S. Pat. No. 5,364,689; U.S. Pat.No. 6,103,361; US 2004/0051297; US 2004/0009309; EP-A-710508, WO02/090002; WO 03/000801; WO 2005/002866, and US 2002/0160194; thesedocuments are incorporated herein by reference.

U.S. Pat. No. 2,418,479 and U.S. Pat. No. 2,570,856 disclose a processand a coating composition for the magnetic orientation of metallicpigments in paint films. Said films have a high degree of orientationand a low degree of brightness, as well as unusual reflectance andtranslucency properties. The metallic pigments comprise flakes offerromagnetic material, preferably nickel, in amounts ranging from 0.1%to 95% by weight of the film forming binder; and volatile organiccomponents are present in the composition in amounts ranging from 50% to70% of the total weight. The wet film is applied in a thickness of 25mils (635 μm), and subject to a magnetic field for orienting themetallic flakes, maintaining the field until the film is dry. Thesedocuments are not concerned with OVI® and disclose merely paintcompositions comprising magnetic metal flake pigments and effectcoatings based thereupon. No formulation rules are given as to therelationship of flake size, flake concentration and coating thicknessfor obtaining the best optical effect.

U.S. Pat. No. 3,791,864 and DE 2006848-A refer to stove-enamelcompositions, nitro-cellulose compositions, and two-componentcompositions comprising magnetic components (e.g. lamellar or rod shapediron pigments), for the production of magnetically oriented coatings.The documents are about a method and a process for the magneticorientation of pigments in two layer coatings; the formulation aspectsof the involved coating composition are not, however, contemplated.

U.S. Pat. No. 3,676,273 discloses a magnetically oriented coating,comprising highly reflective nickel flakes dispersed in an acrylicbinder. The amount of magnetic pigment ranges form 0.1% to 95% by weightof the film forming material. Specific formulation aspects are notaddressed in this document.

U.S. Pat. No. 5,364,689 discloses a painted product comprising magneticnon-spherical particles in a paint medium, wherein the said magneticparticles are oriented to yield a pattern of a three-dimensional opticalappearance. The magnetic particles comprise one or more of nickel,cobalt, iron, and their alloys. The particles have a thickness of 0.1 to1.0 μm and a length of 10 to 100 μm. The paint medium is selected fromalkyd, polyester, acrylic, polyurethane, and vinyl resins. The particlesare present in amounts between 1 and 60 parts per 100 parts by weight ofthe paint medium. No particular rules concerning the formulation of thepaint medium are given, however.

U.S. Pat. No. 6,103,361 refers to heat resistant coating compositionscontaining fluoropolymers, such as PFTE (poly-tetrafluoroethylene) andmagnetizable flakes, which allow for the magnetically inducing of apattern in the coating of a frying pan.

US 2004/0051297 and US 2004/0009309 refer to a method and an apparatusfor orienting magnetic flakes during a painting or printing process.Particles of a magnetic optically variable pigment are dispersed in aliquid paint or ink medium. The typical flake is about 20 μm large andabout one μm thick. The flakes typically include a magnetic metal layersuch as a thin film of ferromagnetic metal or alloy, such as cobalt,nickel or PERMALLOY (typically 80% Ni, 20% Fe) and an opticalinterference structure, such as an absorber-dielectric-reflectorFabry-Perot type structure on both sides of the metal layer. US2004/0051297 contains a remark as to the influence of film thickness andthe type of organic carrier used on the magnetic orientability of thepigments. However, no further details concerning the best formulation ofthe coating composition for the application purpose are disclosed.

WO 02/090002 refers to methods for producing imaged coated articles byusing magnetic optically variable pigment, as well as to coatedarticles. The pigment consists of reflective magnetic flakes (RMF) ofthe type described in WO 03/000801 “MultiLayered Magnetic Pigments andFoils” and has a magnetic core layer. No formulation rules are given,however, as to the coating composition which should be used.

WO 05/002866 refers to a method and means for producing a magneticallyinduced design in a coating containing magnetic particles. Said coatingcomprises preferably magnetic optically variable particles. The coatingcomposition is preferably selected from the group of liquid inks,comprising screen-printing inks, gravure inks and flexographic inks.Liquid inks have low viscosity (in the range of 0.1 to 5 Pa*s at 20° C.)and allow for an easy orientation of the magnetic pigment. Drying/curingof the ink can be based on solvent or water evaporation, as well as onUV-crosslinking or on hybrid curing mechanisms, including evaporation ofdiluents, UV-curing and other reticulation reactions, such asoxypolymerization cross-linking reactions. None of the given inkformulas were optimized, however, with respect to the magneticimage/effect imprinted in the coating.

US 2002/0160194 refers to multi-layered magnetic pigments and foils. Thedisclosed pigment flakes can be interspersed within a binder medium toproduce a colorant composition (ink) which can be applied to a widevariety of objects or papers. The binder medium contains a resin ormixtures of resins, and a solvent (organic solvent or water), and can bedried/cured by thermal processes such as thermal cross-linking, thermalsetting, or thermal solvent evaporation, or by photochemicalcross-linking.

The optically variable inks and coating compositions used in the art areaimed at exhibiting a bright color, a strong color shift and yielding agood substrate coverage using an as low as possible amount of opticallyvariable pigment. A low pigment concentration is desirable to limit theraw material costs, as well as to obtain a good printability of the inkand durability of the print. These objectives are achieved by providingthe printing inks with a relatively high amount of volatile componentssuch as organic solvents, water or mixtures thereof, of the order of 50%or more, per weight of the composition, and a relatively low amount ofnon-volatile components, i.e. the binder medium and the OVP, of theorder of 50% or less, per weight of the composition.

This particular formulation ensures a volume reduction of the coatinglayer during the drying process and a corresponding orientation of theOVP particles in the plane of the printed substrate. This is why mostOVI formulations or coating formulations containing optical effectpigments are solvent or water based, with solid contents not exceeding50%. The solid content represents the part of non-volatile compounds ofa printed ink or coating layer after the drying/curing process.

In the case of magnetic optically variable pigments however, it has beenfound that this type of ink formulations, when used for the magneticinduction of images, patterns or designs in the printed ink layer, leadsto poor visual effects.

SUMMARY OF THE INVENTION

The technical problem underlying the present invention was to findcoating compositions and corresponding formulation rules which areparticularly adapted for the magnetic orientation of magnetic opticallyvariable pigment (MOVP) in a printed ink or coating layer, yielding anattractive visual effect. With conventional formulations, suited forprinting OVI®, the magnetic images transferred into the wet ink film arenoteworthy strongly diminished in resolution and contrast during thedrying/curing process, due to the vertical shrinking of the printed inkor coating layer.

The resulting inks should be compatible with standard printingrequirements such as drying speed and printing resolution, as well aswith economical constraints to control cost by limiting the appliedquantities. The printing techniques to be used to print MOVP particlesshould be (copperplate) intaglio-, flexography-, gravure-,screen-printing as well as roller coating.

According to the present invention, this problem has been solved by acoating composition as defined in the appended claims.

In particular, the present invention is related to a coating compositionfor producing a magnetically induced image according to the presentinvention therefore consists of volatile components (S) and non-volatilecomponents, the latter consisting of an ink vehicle (I) and magneticallyorientable optically variable interference pigment (P), characterized inthat the ratio of the volume of the ink vehicle (V(I)) to the volume ofthe pigment (V(P)) is higher than 3.0, preferably higher than 4.0, andmost preferably higher than 5.0.

The present invention is also related to a process for manufacturing acoating composition for producing a magnetically induced image, whichcomprises the step of mixing together volatile components (S), andnon-volatile components, the latter consisting of an ink vehicle (I) andmagnetically orientable optically variable interference pigment (P),characterized in that the ratio of the volume of the ink vehicle (V(I))to the volume of the pigment (V(P)) is higher than 3.0, preferablyhigher than 4.0, and most preferably higher than 5.0.

According to the present invention, the term “magnetic opticallyvariable pigment (MOVP)” refers to platelet or flake shaped magneticpigment particles carrying an optical interference coating, as known inthe art. The particular characteristic of MOVP with respect to OVP isthat the MOVP particles can be oriented by an applied magnetic field.MOVP are thus “magnetically orientable optically variable interferencepigments”. The MOVP comprised in the printing ink or coating compositionof the present invention consists of flat platelet or flake shapedparticles chosen from the group of vacuum deposited magnetic thin filminterference pigments, interference coated metallic pigments,interference coated nonmetallic pigments, magnetic liquid crystalpigments as disclosed in PCT/EP2005/056260, and mixtures thereof.Particularly preferred are the five-layer or the seven-layer vacuumdeposited thin film interference pigments of U.S. Pat. No. 4,838,648 andWO 02/73250.

The MOVP used in the present invention is also characterized by itsaverage particle size. In order to get saturated colours and abruptcolour changes, the mean particle diameter (d50) should typically rangefrom 5 to 40 μm, preferably from 15 to 25 μm, and have a thickness inthe range of 0.1 to 6 μm, more preferably in the range of 0.5 to 3 μm.

According to the present invention, the term “Volatile components”refers to components having a boiling point below 300° C. at ambientpressure, i.e. everything which eventually evaporates after printing.The volatile components present in the coating/ink composition can bechosen from organic solvents, water and mixtures thereof, i.e. fromsolvents typically used for making printing inks.

According to the present invention, the term “non-volatile components”refers to components having a boiling point of at least 300° C. atambient pressure, i.e. everything which remains after printing.

According to the present invention, the term “ink vehicle” refers to thenon-volatile part of a printing ink or coating composition, except themagnetic optically variable interference pigment. The ink vehicle may,however, comprise other pigments. Thus, the ink vehicle according to thepresent invention may comprise components from the group consisting ofvarnishes (i.e. binders), oligomers, fillers, pigments, dyes, levelingagents, wetting agents, surfactants, corrosion inhibitors, dryingcatalysts, photo initiators, waxes, cross-linking agents, non-volatilediluents or monomers.

According to the present invention, the term “volume of the ink vehicle”refers to the volume of the dried/cured ink vehicle.

According to the present invention, by the term “drying”, threedifferent mechanisms are commonly addressed in the art. Two merelyphysical drying processes refer to the evaporation of volatilecomponents from the printed ink or coating, leaving back its solid resinand pigment components, and to the penetration/absorption ofnon-volatile ink or coating solvent into the substrate. A third,chemical drying process, also called hardening or curing, refers to thetransformation of a liquid composition into a solid one through achemical polymerization or cross-linking reaction initiated by UVradiation, electron beam or oxypolymerization (oxidative reticulationinduced by the joint action of oxygen and catalysts, such as Co and Mncatalysts). One or more of these drying processes may be implied in thedrying of a same particular printing ink or coating. Thus, curing is aspecific embodiment of drying. “Dual Curing” means a combination ofphysical evaporation and/or penetration in the substrate of volatilecomponents with UV curing or oxypolymerization or chemicalpolymerisation initiated with an appropriate additive; “UVOX” means acombination of UV curing and oxypolymerization.

The printing techniques which can be used to print MOVP particles are(copperplate) intaglio-, flexography-, gravure-, screen-printing as wellas roller coating.

To achieve the printing requirements, the corresponding printingelements are selected so as to deposit typical average dry ink filmthickness in the range of 2 to 50 μm, preferably of 5 (flexography) to30 μm (intaglio- or screen-printing).

The average diameter of the pigment is selected in function of the layerthickness which can be typically obtained and of the technicalconstraints linked to a given printing application. Selecting too smallpigments will, in all cases, result in poor colour shifting, stronglight scattering and low chroma. This is well-known to the skilled manand will be taken into consideration by him when selecting theappropriate pigments.

It was found that a vertical shrinking of the printed ink or coatinglayer during the drying/curing process must be avoided, in order toprevent the oriented pigment particles from adopting a flat position,which significantly reduces or even completely destroys the orientationeffect produced by the magnetization. This is achieved by providing asufficiently thick layer of non-volatile ink vehicle which will remainafter evaporation of the volatile components.

Thus, of paramount importance is the volume ratio V(I)/V(P) of thedried/cured ink vehicle (I) to that of the magnetic optically variablepigment (P) present in the ink vehicle. It was found that below a volumeratio V(I)/V(P) of 3.0 it was impossible to produce a satisfactorymagnetically induced image in a coating of the present invention.According to the present invention, Volume ratios are calculated basedon experimental data and known product characteristics, as disclosedhereinafter in the detailed description.

What has to be considered is the thickness of the dried/cured ink layer.Inventors have found that the dried/cured solid ink layer should not beless in thickness than d50/3, preferably not less than d50/2, in orderto obtain an orientable coating layer yielding a satisfactorymagnetically oriented image. The quantity d50 is the mean diameter ofthe magnetic optically variable pigment and determined as known in theart.

In coatings, which are significantly thinner than d50/2, the achievableorientation effect is poor.

The coating composition of the present invention for producing magneticimages may be a copperplate intaglio printing ink, a flexographicprinting ink, a gravure printing ink, a silkscreen printing ink, or aroller coating ink, and can be correspondingly used in a copperplateintaglio-, a flexographic-, a gravure-, a silkscreen-printing or aroller-coating process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 to 3 show (a) the different optical effects obtained with thethree exemplary compositions, and (b) the resulting pigment orientationinside the ink layers.

FIG. 1 shows the results obtained with a UV cured screen printing inkaccording to example 2a. In FIG. 1 a, the resulting magnetized image isshown. In FIG. 1 b, a Scanning electron microscope (SEM) cross sectionof the ink layer on the substrate is shown.

FIG. 2 shows the results obtained with a UV cured screen printing inkaccording to example 2b. In FIG. 2 a, the resulting magnetized image isshown. In FIG. 2 b, a Scanning electron microscope (SEM) cross sectionof the ink layer on the substrate is shown.

FIG. 3 shows the results obtained with a UV cured screen printing inkaccording to example 2c. In FIG. 3 a, the resulting magnetized image isshown. In FIG. 3 b, a Scanning electron microscope (SEM) cross sectionof the ink layer on the substrate is shown.

DETAILED DESCRIPTION OF THE INVENTION

The magnetic optically variable printing inks or coating compositions ofthe present invention are divided into three main constituents. Themagnetic optically variable pigment (P), the solvents or volatilecomponents (S), (i.e. everything which eventually evaporates afterprinting: organic solvents, water or mixtures thereof), and the inkvehicle (I), (i.e. everything which remains after printing, except thepigment: i.e. non-volatile components such as varnishes, oligomers,fillers, pigments, dyes, leveling agents, wetting agents, surfactants,corrosion inhibitors, drying catalysts, photo initiators, waxes,cross-linking agents, non-volatile diluents or monomers).

The magnetic pigment of the present invention is typically chosen such,that the platelet shaped particles have a mean diameter (d50) in therange of 5 to 40 μm, more preferably the diameter is in the range of 15to 25 μm, and a thickness in the range of 0.1 to 6 μm, more preferablyin the range of 0.5 to 3 μm.

The diameter of the platelet shaped particles is to be understood as themean (d50) value, determined as known to the skilled man. Similarly, thethickness of the platelet shaped particles is to be understood as themean of the distance between the platelets' upper and lower surfaces,determined as known to the skilled man.

It was found by the inventors, that, independent of the process used forapplying the magnetic ink to the substrate, there is a lower limit tothe volume ratio V(I)/V(P) of the ink vehicle (I) to the magneticoptically variable pigment (P) in the ink, under which the visual effectof the magnetically oriented images, patterns or designs fades away upondrying of the wet film. This lower limit was found to be at a volumeratio V(I)/V(P) of 3.0. Good results are obtained with a volume ratioV(I)/V(P) higher than 4.0, preferably higher than 5.0.

In other words, there must be sufficient volume of ink vehicle (I) pervolume of pigment (P) in the dry ink film, in order to allow the pigmentflakes to keep their orientations, if this latter is not in the plane ofthe substrate.

For calculating the volume ratio V(I)/V(P), the volume of the inkvehicle alone, and the volume of the pigment in the ink vehicle must beknown. This will be outlined hereinafter with reference to example 2.

Three ink formulations according to example 2a to 2c (UV-curingsilkscreen printing inks) served as a basis for correlating formulationparameters with the obtainable magnetic orientation effect. With respectto the details of those examples, reference is made to the experimentalsection below. The formula of example 2a showed excellent magneticorientability; that of example 2b showed some degradation, as comparedto example 2a, and that of example 2c showed serious degradation, ascompared to example 2a. It can be inferred that formulas with a ratioV(I)/V(P) lower than 3.0 will no longer show any useful effect.

Experimental data determined from the base formula of example 2 (firstcolumn) showed that the specific weight (density) of the wet ink(D_(inkw)) is 1.24 g/cm³, (D_(inkd) 1.26 g/cm³ for the dry ink). Thedensity of the magnetic optically variable pigment was determined to be2.82 g/cm³ (the pigment density varies to a certain extent, depending onthe ratio of dielectric material (MgF₂ (optical properties)) and ofmagnetic material (Ni, Fe, Co or Ni alloy (magnetic properties)) in thepigment flake. The pigment used in the present example had anexperimental density of 2.82 g/cm³), and the density of the solvent(Dowanol) is 0.967 g/cm³. The experimental densities were determined bymeans of a pycnometer. The use of a pycnometer for determining densitiesis well-known to the skilled man and needs not be discussed in detailhere (cf. standard ISO 1183-1:2004).

The base wet ink formula can be approximately described as follows(wherein W=weight, V=volume before mixing, I=ink vehicle, P=MOVP,S=solvent, D=density):

$\begin{matrix}{{{W(I)} + {W(S)} + {W(P)}} = {D_{inkw}\left( {{V(I)} + {V(S)} + {V(P)}} \right)}} \\{= {{1.241\left\lbrack \frac{g}{{cm}^{3}} \right\rbrack}*{\left( {{V(I)} + {V(S)} + {V(P)}} \right)\mspace{14mu}\left\lbrack {cm}^{3} \right\rbrack}}}\end{matrix}$

Taking into account the per weight formulation ratio W(I)+W(S)=0.80 g/gand W(P)=0.20 g/g and the density of the pigment D(P)=2.82 g/cm³, thepigment volume V(P) per gram of ink is calculated by means of the knowncorrelation d=m/V as 0.071 cm³ and the remainder volume of the wet inkvehicle is 0.735 cm³.

The dried/cured ink formulation can be approximately described asfollows:

$\begin{matrix}{{{W(I)} + {W(P)}} = {D_{inkd}^{*}\left( {{V(I)} + {V(P)}} \right)}} \\{= {{1.26\left\lbrack \frac{g}{{cm}^{3}} \right\rbrack}*{\left( {{V(I)} + {V(P)}} \right)\mspace{14mu}\left\lbrack {cm}^{3} \right\rbrack}}}\end{matrix}$

From the data above and the density of the solvent D(S)=0.967 g/cm³(Dowanol), the quantity of evaporated solvent can be determined, and theweight of the ink vehicle results as 0.747 g/g of the original inkformulation; the corresponding volume V(I) being 0.68 cm³; this gives adensity of the ink vehicle D(I) of 1.098 g/cm³. In the present example(base formulation), the volume ratio V(I)/V(P) is thus given as 9.58.

Referring to the magnetic orientation examples (formulations of examples2a to 2c) the corresponding volume ratios were calculated in the sameway, using the determined specific weights of the MOVP and the inkvehicle (I), and the corresponding weight percentages:

TABLE 1 Formulation Ex. 2a Ex. 2b Ex. 2c Ink vehicle 80 60 30 0.7290.546 0.273 Pigment 20 20 20 V(P) 0.071 0.071 0.071 solid content 100 8050 volume ratio 10.27 7.69 3.85 V(I)/V(P) dry film 19 14 11 thickness

In analogy the volume ratio V(I)/V(P) of example 1, which refers to anintaglio printing ink, was determined to 4.83. The densities of thedried/cured and the wet printing ink were determined as D_(inkd)=1.37g/cm³ and D_(inkw)=1.236 g/cm³. The densities of the pigment and thesolvent were determined as D(P)=2.82 g/cm³, and D(S)=0.805 g/cm³ (Inksolvent 27/29 Shell industrial chemicals). 0.3 g/g of pigment (P) weremixed with 0.7 g/g of (ink vehicle (I)+ink solvent (S)). The density ofthe ink vehicle D(I) was determined to 1.066 g/cm³.

A further point to consider for a successful ink formulation is thethickness of the dried/cured solid coating layer. The coating layershould be thicker than d50/3, preferably thicker than d50/2, wherein d50is the mean diameter of the magnetic optically variable pigment flakes,determined as known in the art.

Coatings containing solvent must be correspondingly thicker uponapplication than coatings which are solvent free; considering that thethickness of the dry and solid layer, after evaporation of the solvent,must fulfill the given criteria. In the examples underlying FIG. 1 toFIG. 3, the dry solid layer thickness fulfils the preferred d50/2criterion in all cases.

The amount of non-volatile components in the coating composition of thepresent invention is chosen between 50% and 100% per weight of the totalcomposition, preferably between 80% and 100% per weight of the totalcomposition.

FIG. 1 refers to a coating composition having a volume ratio V(I)/V(P)of 10.3 and the maximal solid content of 100% per weight (20% of MOVP),which results in optimal magnetically induced effects (images, patternsor designs) in a corresponding coating layer, whereas FIG. 3 refers to acoating composition having a volume ratio V(I)/V(P) of 3.8 and a solidcontent of 50% per weight (20% MOVP), which still allows to producemagnetically induced images, patterns or designs in the coating layer,albeit of low quality.

The skilled person will recognize that the general concept outlined inpresent description is applicable to a series of printing inkformulations with different amounts of solid contents (with differentamounts of MOVP), resulting in different volume ratio of V(I)/V(P).

Table 2 is one possible compilation out of many of a calculated matrixof formulations according to the inventive concept. Volume ratios rangebetween around 0.6 and around 23.

TABLE 2 (UV curing silkscreen ink according to example 2) SC 100% SC 90%SC 80% SC 50% I [%] 90 80 70 40 P [%] 10 10 10 10 V(I) 0.8197 0.72860.6375 0.3643 [cm³/cm³] V(P) 0.0355 0.0355 0.0355 0.0355 (cm³/cm³]V(I)/V(P) 23.1148 20.5464 17.9781 10.2732 I [%] 80 70 60 30 P [%] 20 2020 20 V(I) 0.7286 0.6375 0.5464 0.2732 [cm³/cm³] V(P) 0.0709 0.07090.0709 0.0709 [cm³/cm³] V(I)/V(P) 10.2732 8.9891 7.7049 3.8525 I [%] 7060 50 20 P [%] 30 30 30 30 V(I) 0.6375 0.5464 0.4554 0.1821 [cm³/cm³]V(P) 0.1064 0.1064 0.1064 0.1064 [cm³/cm³] V(I)/V(P) 5.9927 5.13664.2805 1.7122 I [%] 60 50 40 10 P [%] 40 40 40 40 V(I) 0.5464 0.45540.3643 0.0911 [cm³/cm³] V(P) 0.1418 0.1418 0.1418 0.1418 [cm³/cm³]V(I)/V(P) 3.8525 3.2104 2.5683 0.6421 SC Solid Content, I = weightfraction of Ink vehicle, P = weight fraction of pigment, V(I)/V(P)volume ratio.

Table 2 provides for an arbitrary selection based on different totalsolid contents and their respective composition. From a technical pointof view, the skilled man knows that for producing satisfying results aminimal amount of pigment is required, and that a high amount of pigmentpresent in the printing ink decreases the printability of the ink andincreases the costs.

Therefore Table 2 provides for the selection of suitable coatingcompositions with respect to the limits of the solid contents as well asto the volume ratio of V(I)/V(P) according to the present invention. Thehighlighted examples correspond to the most preferred ratio V(I)/V(P)higher than 5.0 of example 2 and serve a mere illustrative purpose.

Printing inks having a volume ratio below 4.0 can be suitable as wellfor carrying out the present invention; however, the elevated amount ofmagnetic optically variable pigment with respect to the ink vehiclerenders the magnetic orientation of the MOVP more difficult may addsunnecessary cost to the magnetic optically variable printing ink, if theprinted ink layer is thick.

The process for manufacturing the printing ink or coating composition ofthe present invention for producing a magnetically induced image,comprises the step of mixing together volatile components (S), andnon-volatile components, the latter consisting of an ink vehicle (I) andof magnetically orientable optically variable interference pigment (P),characterized in that the ratio of the volume of the ink vehicle (V(I))to the volume of the pigment (V(P)) is higher than 3.0, preferablyhigher than 4.0, more preferably higher than 5.0.

The volatile components for the process of producing a printing ink orcoating composition of the present invention are chosen from organicsolvents, water, and mixtures thereof.

The present invention is furthermore related to a process of producing amagnetically induced image, which comprises the steps of a) applying acoating composition according to the present invention to a substratesurface, b) orienting the magnetic pigment particles in the appliedcoating composition of step a) by applying a magnetic field, and c)curing/drying the oriented coating composition of step b) to fix theparticles in the oriented positions.

The coating process of step a) for applying the coating composition to asubstrate surface is preferably chosen from engraved copperplateintaglio printing, flexographic printing, gravure printing, rollercoating, and silkscreen printing. These processes are well-known to theskilled man.

Said printing processes allow for the deposit of a wet film-layer fromabout 2 μm to about 50 μm. The preferred wet film deposit is in therange from about 5 μm to about 30 μm. The resulting coating on thesubstrate has an average dry layer thickness comprised between 2 μm andless or equal than 50 μm, preferably between 5 μm and less or equal than30 μm, more preferably between 10 μm and less or equal 20 μm.

The orienting step b) can be performed either simultaneously with thecoating step a) or subsequently to the coating step a). Magneticorientation of magnetic particles is described and known in the art.Reference is made in this respect to the prior art documents cited inthe introductory portion of the present application.

The curing/drying step c) can be performed by physical evaporation ofvolatile compounds, UV-curing, oxidative reticulation, chemicalcross-linking, electron beam curing, or by any combination thereof. Alsothis step is known in the art and does not have to be described here indetail.

The present invention is furthermore related to a magnetically inducedimage coating on a substrate which comprises magnetically orientedpigment (P) in a cured solid ink vehicle (I), characterized in that theratio of the volume of the ink vehicle (V(I)) to the volume of thepigment (V(P)) is higher than 3.0, preferably higher than 4.0, and mostpreferably higher than 5.0, and that the coating layer is thicker thand50/3, preferably thicker than d50/2, wherein d50 is the mean diameterof the magnetically orientable optically variable interference pigmentflakes. The amount of magnetically oriented pigments in the cured solidink vehicle (I) ranges between 1% to 40% per weight, preferably between5% to 30% per weight, more preferably between 10-20% per weight of thewet coating.

The printing ink or coating composition of the present invention can beused for producing magnetically induced images. Said magneticallyinduced images can be used as security elements for e.g. banks notes,credit cards, access cards, security badges, documents of value, rightsor identity, transportation tickets, lottery tickets, event tickets, taxbanderoles, security threads, labels, foils, strips or product securityapplications. Thus, the present invention is also related to the use ofthe herein disclosed coating compositions for the above applications, aswell as to a security document comprising a magnetically induced imageobtained with a coating composition of the present invention.

Said security element may further comprise additional marking means suchas infrared markers, fluorescent markers, UV markers, phosphorescentmarkers, magnetic markers, forensic markers and mixtures thereof.

The invention can be practiced on any type of printable sheet or webmaterial, in particular on materials used for producing e.g. banksnotes, credit cards, access cards, security badges, documents of value,rights or identity, transportation tickets, lottery tickets, eventtickets, tax banderoles, security threads, labels, foils, strips orproduct security applications. The printable sheet or web material mayfurther comprise a single layer, as well as a plurality of layers.

The present invention will now be further described by reference tonon-limiting examples and drawings. Unless indicated otherwise, allamounts are given as weight percents.

Example 1 Intaglio Paper-Wipe Ink

In this example, a formulation of an intaglio paper-wipe ink accordingto the present invention is given.

Addition product of tung oil and maleic acid   35% modified phenolicresin in a high boiling mineral oil (PKWF 28/31) Long oil alkyd resin7.50% Alkylphenolic resin modified with raw tung oil in   16% inksolvent 27/29 Polyethylene wax 3.30% Aerosil 200 (Degussa-Huels) 2.00%Optically variable pigment magnetic (7 layers)   30% Ink solvent 27/29(Shell Industrial Chemicals)   6% Cobalt octoate (11% metal) 0.10%Manganese octoate (10% metal) 0.10%

Example 2 Silkscreen Ink, UV-Drying

In this example, a formulation of a silkscreen ink according to thepresent invention is given.

Base Ex. Ex. Ex. Formula 2a 2b 2c Ink vehicle (I) total:  70% 80%  60% 30%  Epoxyacrylate oligomer 40  45.7  34.2  17.1  Trimethylolpropanetriacrylate mo- 10  11.5  8.6 4.3 nomer Tripropyleneglycol diacrylatemo- 10  11.5  8.6 4.3 nomer Genorad 16 (Rahn) 1 1.1 0.9 0.4 Aerosil 200(Degussa-Huels) 1 1.1 0.9 0.4 Irgacure 500 (CIBA) 6 6.8 5.1 2.6 GenocureEPD (Rahn) 2 2.3 1.7 0.9 Pigment (P) total:  20% 20%  20%  20%  Magneticoptically variable pig- 20  20    20    20    ment (7 layers) Solvent(S) total:  10% 0% 20%  50%  Dowanol PMA 10  0    20    50   

The formulation of example 2a was applied on security paper. Prior todrying the wet ink was magnetized. The average dry film thickness was 19μm. The solid content of the ink was 100%, wherein the magneticoptically variable pigment (P) was present in an amount of 20% perweight and the ink vehicle (I) was present in an amount of 80% perweight. The result is shown in FIGS. 1 a and 1 b. In FIG. 1 a, theresulting magnetized image is shown. In FIG. 1 b, a Scanning electronmicroscope (SEM) cross section of the ink layer on the substrate isshown. The average angle of the flakes is measured as being 24°+/−12°from the horizontal.

In FIG. 1 b, “Matrix” refers to the embedding, which is needed for thepreparation of cross sections. The effect produced by the magnetizationis a clearly defined image.

The formulation of example 2b was applied on security paper. Prior todrying the wet ink was magnetized. The average dry film thickness was 14μm. The solid content of the ink was 80%, wherein the magnetic opticallyvariable pigment (P) was present in an amount of 20% per weight, the inkvehicle (I) was present in an amount of 60% per weight, and the solvent(S) was present in an amount of 20% per weight. The result is shown inFIGS. 2 a and 2 b. In FIG. 2 a, the resulting magnetized image is shown.In FIG. 2 b, a SEM cross section of the ink layer on the substrate isshown. The average angle of the flakes is measured as being 25°+/−12°from the horizontal. The effect produced by the magnetization is animage of diminished resolution, but still quite clearly visible.

The formulation of example 2c was applied on security paper. Prior todrying the wet ink was magnetized. The average dry film thickness was 11μm. The solid content of the ink was 50%, wherein the magnetic opticallyvariable pigment (P) was present in an amount of 20% per weight, the inkvehicle (I) was present in an amount of 30% per weight, and the solvent(S) was present in an amount of 50% per weight. The result is shown inFIGS. 3 a and 3 b. In FIG. 3 a, the resulting magnetized image is shown.In FIG. 3 b, a SEM cross section of the ink layer on the substrate isshown. The average angle of the flakes is measured as being 10°+/−9°from the horizontal. The effect produced by the magnetization is animage of rather poorly defined resolution.

The formula of example 2a showed excellent magnetic orientability; thatof example 2b showed some degradation, as compared to example 2a, andthat of example 2c showed serious degradation, as compared to example2a. It can be inferred that formulas with a ratio V(I)/V(P) lower than3.0 will no longer show any useful effect.

It is evident from FIGS. 1 to 3 and from Table 1 above, that anexcellent correlation exists between the orientability of the magneticpigment and the volume ratio V(I)/V(P) on one hand, and between theorientability of the pigment and the dry film thickness on the otherhand.

Example 3 Silkscreen Ink, UV-Drying

In this example, a formulation of silkscreen ink according to thepresent invention is given.

Epoxyacrylate oligomer 40% I Trimethylolpropane triacrylate monomer 10%Tripropyleneglycol diacrylate monomer 10% Genorad 16 (Rahn)  1% Aerosil200 (Degussa-Huels)  1% Irgacure 500 (CIBA)  6% Genocure EPD (Rahn)  2%Optically variable pigment magnetic (5 layers) 20% P Dowanol PMA 10% S

Example 3 Flexography Ink, UV-Drying

In this example, a formulation of flexographic ink according to thepresent invention is given.

Urethane acrylate oligomer 40% Glycerol propoxylated triacrylate monomer10% Tripropyleneglycol diacrylate monomer 15% Florstab UV-1 (Kromachem) 1% Optically variable pigment magnetic (7 layers) 25% Aerosil 200(Degussa-Huels)  1% Irgacure 500 (CIBA)  6% Genocure EPD (Rahn)  2%

1. Magnetically induced image coating on a substrate comprisingmagnetically oriented optically variable interference pigment (P) in acured solid ink vehicle (I), wherein the ratio of the volume of the inkvehicle (V(I)) to the volume of the pigment (V(P)) is higher than 5.0,and that the coating layer is thicker than d50/3, wherein d50 is themean diameter of the magnetically orientable optically variableinterference pigment flake, and that said image coating is printed witha printing ink selected from the group consisting of flexographicprinting inks, gravure printing inks, silkscreen printing inks androller coating inks.
 2. Magnetically induced image coating according toclaim 1, wherein said coating layer is thicker than d50/2. 3.Magnetically induced image coating according to claim 1, wherein theprinting ink consists of volatile components (S) and nonvolatilecomponents, the non-volatile components consisting of an ink vehicle (I)and magnetically orientable optically variable interference pigment (P),wherein the ratio of the volume of the ink vehicle (V(I)) to the volumeof the pigment (V(P)) is higher than 5.0.
 4. Magnetically induced imagecoating according to claim 3, wherein said non-volatile components arepresent in an amount greater than 50% per weight of the totalcomposition.
 5. Magnetically induced image coating according to claim 3,wherein said non-volatile components are present in an amount greaterthan 80% per weight of the total composition.
 6. Magnetically inducedimage coating according to claim 3, wherein said volatile components areselected from the group consisting of organic solvents, water, andmixtures thereof.
 7. Magnetically induced image coating according toclaim 3, wherein said magnetically orientable optically variable pigmentis selected from the group consisting of vacuum deposited magnetic thinfilm interference pigments, interference coated metallic pigments,interference coated non-metallic pigments, magnetic liquid crystalpigments, and mixtures thereof.
 8. Magnetically induced image coatingaccording to claim 7, wherein said magnetically orientable interferencepigment is selected from the group consisting of five-layer andseven-layer vacuum deposited magnetic thin film interference pigment. 9.Magnetically induced image coating according to claim 3, wherein saidmagnetically orientable interference pigment has a mean diameter d50 inthe range of 5 μm to 40 μm.
 10. Magnetically induced image coatingaccording to claim 3, wherein said magnetically orientable interferencepigment has a mean diameter d50 in the range of 15 μm to 25 μm. 11.Magnetically induced image coating according to claim 3, wherein saidmagnetically orientable interference pigment has a thickness in therange of 0.1 μm to 6 μm.
 12. Magnetically induced image coatingaccording to claim 3, wherein said magnetically orientable interferencepigment has a thickness in the range of 0.5 μm to 3 μm.
 13. Method ofproducing a magnetically induced image coating according to claim 1 onsubstrates associated with bank notes, credit cards, access cards,security badges, documents of value, rights or identity, transportationtickets, lottery tickets, event tickets, tax banderoles, securitythreads, labels, foils, strips or product security applications,comprising applying said magnetically induced image coating onto therespective substrate.
 14. Method according to claim 13, wherein theprinting ink consists of volatile components (S) and nonvolatilecomponents, the non-volatile components consisting of an ink vehicle (I)and magnetically orientable optically variable interference pigment (P),wherein the ratio of the volume of the ink vehicle (V(I)) to the volumeof the pigment (V(P)) is higher than 5.0.
 15. Method according to claim14, wherein said non-volatile components are present in an amountgreater than 50% per weight of the total composition.
 16. Methodaccording to claim 14, wherein said non-volatile components are presentin an amount greater than 80% per weight of the total composition. 17.Method according to claim 14, wherein said magnetically orientableoptically variable pigment is selected from the group consisting ofvacuum deposited magnetic thin film interference pigments, interferencecoated metallic pigments, interference coated non-metallic pigments,magnetic liquid crystal pigments, and mixtures thereof.
 18. Securityelement for bank notes, credit cards, access cards, security badges,documents of value, rights or identity, transportation tickets, lotterytickets, event tickets, tax banderoles, security threads, labels, foils,strips or product security applications, comprising a magneticallyinduced image coating according to claim
 1. 19. Security elementaccording to claim 18, further comprising marking elements selected fromthe group consisting of infrared markers, fluorescent markers, UVmarkers, phosphorescent markers, magnetic markers, forensic markers andmixtures thereof.
 20. Bank notes, credit cards, access cards, securitybadges, documents of value, rights or identity, transportation tickets,lottery tickets, event tickets, tax banderoles, security threads,labels, foils, strips or product security applications, comprising asecurity element according to claim 18.